1. Field of the Invention
This invention relates to methods of recording and reproducing audio signals, and more particularly to methods of recording and reproducing digital audio signals in association with digital video signals using a digital video tape recorder (VTR).
2. Description of the Prior Art
It is known to record digital audio signals in oblique tracks on a magnetic tape together with digital video signals using a digital VTR. In such a case, it is necessary to provide synchronization of the digital audio signals, because the rotational frequency of the head drum of the digital VTR is locked to the digital video signals. This rotational frequency is normally 150 Hz, so three rotations of the drum and hence three head assembly scans correspond to one field of a 625-line 50-field per second television system, and likewise two-and-a-half head scans correspond to one field of a 525-line 60-field per second television system.
The recommended sampling frequency for a digital audio signal is 48 KHz and if therefore the drum rotational frequency were exactly 150 Hz, this would correspond to an integral number of audio samples per head scan. If, further, the clock controlling the audio sample rate where locked to the rotational frequency of the drum, there would be no problem with audio synchronization.
There are, however, two situations in which the above assumptions do not hold true. Firstly, in 525-line 60-field per second operation using the NTSC system, the field frequency is not in fact precisely 60 fields per second, but has an off-set of one part in a thousand. In consequence, the drum rotational frequency is not precisely 150 Hz, but is 149.85 Hz. This figure will not divide integrally into 48 KHz.
To overcome this problem it has been proposed that the audio sampling frequency of 48 KHz should likewise be off-set by one part in a thousand, but clearly it is inconvenient to use different sampling frequencies for the audio signals of different television systems.
Secondly, the audio sample rate may not in fact be precisely synchronized to the drum rotational frequency, due to imperfect locking of the frequencies of the various systems within a television studio. The same problem arises with the video signals, but it is easily overcome in that case by use of a field store or a frame store so that periodicaly a field or a frame can be omitted or repeated to restore synchronization. A similar technique cannot of course be used for the audio signals without severe degradation of the reproduced sound.